Flash memory was originally developed as a derivative of Erasable Programmable Read Only Memory (EPROM). Conventional EPROM technology uses hot electron injection (also called avalanche injection) to program the memory and ultraviolet (UV) light to erase the contents of the memory. Avalanche injection of electrons into the floating gate is achieved by applying high positive voltage to both the drain and the control gate, and grounding the source. Exposing the cell to UV light increases the energy of the floating gate electrons to a level where they may jump the energy barrier between the floating gate and the oxide.
Conventional single-transistor cell flash memory technology is similar to single-transistor cell EPROM technology. However, flash memory allows for electrical erasure of the contents of the memory, either of the entire memory array at once or of a sector of the memory at once, by way of cold electron tunneling (also called Fowler-Nordheim tunneling).
An example of a conventional single-transistor cell for flash memory is illustrated in FIG. 1. Such a flash memory cell typically has thinner oxide under the floating gate (between the floating gate (106) and the channel) than an EPROM cell has. The thinner oxide allows for erasure to be achieved via cold electron tunneling between the floating gate (106) and the source (104).
Like programming of EPROM, programming of conventional single-transistor cell flash memory is typically performed by applying high positive voltage to both the drain (102) via the bitline and the control gate (108) via the wordline, while grounding the source (104). This causes hot electron injection from the substrate (101) near the drain (102) to the floating gate (106). This programming by way of hot electron injection is crude in that the charge stored in the floating gate (106) is difficult to control precisely. This inability to control precisely the charge stored in the floating gate (106) is a first disadvantage of conventional single-transistor cell flash memory. This disadvantage makes it difficult to store multi-levels (i.e. more than one bit of information) in a flash cell.
Erasure of conventional single-transistor cell flash memory may be performed by applying a high positive voltage (for example, plus 12 volts) to the substrate (101)and grounding the control gates (108) in a sector. This causes the tunneling of the electrons from the floating gates (106) to the sources (104). Portions of the memory smaller than a sector cannot be erased because the common substrate is shared by all cells in a sector. The size of a sector may be, for example, 512 kilobits of cells for a 4 megabit flash memory organized into 8 sectors. Thus, the inability to erase portions of the memory smaller than a sector is a second disadvantage of conventional single-transistor cell flash memory.
As an alternative to using single-transistor cells, conventional flash memory may instead utilize cells with two or more transistors. For example, each cell may include two transistors: one being a select transistor; and the other being a storage transistor. Utilizing such multiple-transistor cells, erasure of portions as small as a single word have been achieved. However, such multiple-transistor cells are substantially larger than single-transistor cells, and hence are not suitable for high density flash memory applications.
Cell size in conventional flash memory is limited by cell punchthrough requirements. Cell punchthrough occurs when the depletion region of the drain junction merges with the depletion region of the source junction. In order to prevent cell punchthrough, a minimum distance is typically required between drain (102) and source (104) along a bitline (or column). The higher the maximum voltage applied to a drain during operation, the larger the minimum distance must be. In this way, cell punchthrough limits the size of cells along the columnar direction, and so is a third disadvantage of conventional single-transistor cell flash memory.